Astrocytes are nonneuronal cells in the brain (glia) that do not generate electrical impulses but communicate by chemical signaling. This communication can be observed under a microscope with fluorescent calcium indicators that glow more brightly when the concentration of calcium increases inside the cell. Astrocytes release adenosine 5'-triphosphate and other cell signaling molecules that excite membrane receptors on other astrocytes to cause an increase in intracellular calcium in the recipient cell. Many of the substances released by astrocytes also excite neurons, and astrocytes have on their own cell membrane many of the same neurotransmitter receptors used by neurons to communicate across synapses. This allows astrocytes to respond to neural impulse activity, communicate among other astrocytes, and influence neuronal communication by taking up or releasing neurotransmitters from synapses.